The present invention relates to a process for the preparation of high-performance magnetic recording medium obtained by coating a magnetic coating material on the magnetic discs that are used as file memories of electronic computers or the like, or on various other nonmagnetic support members, and particularly it relates to a process for the preparation of magnetic recording medium having a magnetic coating layer of a thickness of about 1 .mu.m.
The trend toward higher recording densities of modern magnetic recording media has required a substantial decrease in the thickness of the recording media of the type of magnetic coating layers, presenting problems of reduced output of electric signals and insufficient resolution at high frequencies. Therefore, increasing demand has been placed in developing the technique of applying more sophisticated magnetic orientation in order to obtain magnetic recording media maintaining high output and excellent resolution.
In order to increase the reproducing outputs of recording, in general, the residual magnetic flux of the magnetic coating layer should be increased. One of the methods for this purpose may be to increase the thickness of the coating layer. The increased thickness of the coating layer, however, deteriorates the characteristics at high frequencies. To improve the high-frequency characteristics, on the other hand, it is necessary to reduce the thickness of the coating layer as well as to increase the coercive force. If the thickness of the coating layer having high coercive force is greatly reduced, the high-frequency characteristics will be improved but sacrificing the characteristics at low frequencies. If the thickness of the coating layer having high coercive force is increased, on the other hand, the magnetic field acts upon the surface areas only at high frequencies, making it difficult to erase the records.
Therefore, ideal recording medium having both of these characteristics consists of providing a thin magnetic coating layer composed of a material having high coercive force on a magnetic coating layer of a relatively large thickness having relatively small coercive force as well as great residual magnetic flux. From this viewpoint, there was proposed a double-coating method which coats such two different layers. Such methods have been disclosed, for example, in Japanese Published Examined Patent Application No. 52-28364. Such methods, however, required complex manufacturing steps resulting in increased manufacturing cost. The double-coating method could be applicable to the magnetic tapes having a coating thickness of about 10 .mu.m, but was not applicable to the coating layers having a thickness of about 1 .mu.m such as magnetic discs for high-density recording. Japanese Published Examined Patent Application No. 39-19281 discloses a method of producing such a magnetic recording medium through one time of coating operation and magnetic orientation. According to this method, a mixture of a magnetic iron oxide powder having a relatively large particle size and the shape of a needle and a magnetic alloy powder having a small particle size and a spherical shape, is coated onto a support member, whereby the dispersion density of the alloy powder having small particle size is increased on the surface areas, in order that a thin layer of alloy powder is coated just on the layer of iron oxide powder. An existing powder of magnetic iron oxide is useable as a magnetic powder having such a large particle size. In many cases, such powders are of the shape of a needle having a length of about 1 .mu.m, a coercive force Hc of 250 to 280 oersteds and a residual magnetization Br of about 500 to 600 gausses. The powder having a small particle size may be a magnetic alloy composed of nickel, cobalt, iron and the like. In many cases, such powder will have a spherical shape of a diameter of smaller than 0.2 .mu.m, a coercive force of greater than 500 oersteds and a residual magnetization of larger than 1500 gausses. A mixture of these two powders (the mixing ratio may be 10 for the former and 1-4 for the latter, though it may vary depending upon the purposes) is coated to a thickness of about 6-13 .mu.m. In this state, the two powders are nearly homogeneously blended together. Under this state, the mixture is subjected to a d-c magnetic field from a magnet placed below the support member before the coated layer is solidified. In this case, the needle-shaped powder having large particle size receives greater attractive force than that of the powder having small particle size, and is collected toward the support member thereby to form a layer on the relatively lower side, while the density of the powder having small particle size is increased toward the upper layer. Thus a coating layer is formed on the support member, consisting of a mixture of the magnetic powder having a relatively large particle size and a small coercive force and the magnetic powder having a relatively small particle size and a large coercive force, wherein the latter powder on the recording surface remote from the support member exhibits greater dispersion density than that of the former powder.
According to the abovementioned method, however, the magnetic field is allowed to be applied only one time. If the magnetic field is applied two or more times repetitively, the oriented powder is disturbed by the second or subsequent application of magnetic field, whereby the surface of the coating layer is coarsened producing the noise level and deteriorating the signal-to-noise ratio. Further, if the magnetic field is repetitively applied by the abovementioned method, the magnetic powder having a relatively small particle size and a high coercive force is finally collected toward the support member, degrading the separation of the two magnetic powders. In the case of a magnetic disc, therefore, even when the magnetic field is oriented using a single set of magnets, the rotation of the magnetic disc renders itself subjected to the magnetic field several times. Because of these reasons, the method disclosed in Japanese Published Examined Patent Application No. 39-19281 is applicable to only the magnetic tapes having a coating layer thickness of about 6 to 13 .mu.m.
Further, in forming a magnetic coating layer of a thickness of about 1 .mu.m on the surface of an aluminum disc or the like by means of the centrifugal coating method, it has also been conventionally performed to adjust the magnetic field orientation for the magnetic powders in the coating layer or to increase the content of magnetic powder to complement the decrease of output due to the reduced thickness of the coating. The object of the magnetic field orientation so far employed, however, was simply to orient the magnetic powder in the direction of reproducing the recording. Moreover, a limitation is imposed on the concentration of magnetic powders in the coating layer, because the increased concentration deteriorates the resolution and decreases the mechanical strength of the coating layer. In general, the content of the magnetic powders is about 50% for the magnetic discs and about 70% for the magnetic tapes.